Add a docker-compose file with celery

[peschmae]

To make the most out of lemur, it's nice to have celery running (so you can use ACME auto-renewal for example).
The current docker-compose file, only starts the lemur, nginx, and psql containers, but it would be nice, to also have a docker-compose file, where the celery beat & workers as well as redis are started as well.

This needs it's own lemur config file, since we need to add the celery & redis configuration, as well as three new pods.

I currently have a setup with a multistage build, where the celery pods, are based on the lemur pod, but without the static files.
As I'm not sure, if it's intended to update the docker-compose version to 3.4, to make use of the multistage build, I'll need to refactor it, to reuse the current lemur file, and just override the start script & command.

[peschmae]

My current implementation can be found at peschmae/lemur-docker/tree/feature/celery